1. Field of the Invention
The present invention relates generally to methods for preparing suspensions of particles and, more specifically, to methods for preparing stable and unstable suspensions of particles and suspensions of particles for coating substrates, and to methods for coating substrates with the suspended particles.
2. Description of the Related Art
Composite materials formed of metallic compounds and reinforcing substrates, such as fibers, are useful in making high-temperature stable structural parts, such as those found in the automotive and aerospace industries. These composite materials have been synthesized using powder metallurgy, thermal spray, and liquid pressure casting techniques. While these techniques produce fairly good composite materials, they are not without their problems. For example, an organic binder is typically used in making the composite materials which must be burned off in the production process. The use of binder not only adds another step in the process, but is a potential source of contamination for the composite material.
Additionally, the above-mentioned techniques for forming composite materials with fibers achieve the best results using large diameter, single-strand fibers while optimum composite structure may require small diameter, multi-strand fibers to insure a good distribution of the fibers.
Some prior art methods which deposit metallic compounds onto fibers, such as arc or plasma spraying, eliminate the need for the binder. However, arc and plasma spraying can be prohibitively expensive and are limited to large diameter, single-strand fibers.
Attempts to produce composite materials formed of NiAl and multi-strand Al.sub.2 O.sub.3 reinforcing fibers by liquid infiltration and pressure casting have had limited success. During these methods, liquid NiAl is unable to adequately wet the Al.sub.2 O.sub.3 fibers, so many of the individual Al.sub.2 O.sub.3 fibers bond together. The areas of fiber-fiber bonding allow the propagation of cracks, thus, decreasing the strength of the composite material.
Ceramic materials are another common material used in the automotive and aerospace industries today. Ceramic materials are typically prepared by processing suspensions of raw material powders into a green body followed by heat treatment, causing densification of the body and microstructural development. Uniformity among the raw material powders in the green body is important as it governs the microstructures of the densified body. Difficulties with forming ceramics arise from the inability of reproducibly making ceramics having identical microstructures and properties. The formation of non-uniform ceramics is caused by inhomogeneities which are a direct result of agglomerates present while the raw material powders are in suspension. The preparation of well-dispersed stable suspensions helps to alleviate the problems caused by agglomeration and leads to the formation of a uniform densified ceramic body.
In addition, new processing methods require alternating from stable suspensions to unstable suspensions (which contain large, loosely-bound agglomerates) or vice versa. Such processing methods prevent segregation of the raw material powders and make it possible to remove undesired ingredients, such as salts and surfactants, by washing.